[PATCH 18/19] rcu/context_tracking: Merge dynticks counter and context tracking states
From: Frederic Weisbecker
Date: Wed Mar 02 2022 - 10:50:19 EST
Updating the context tracking state and the RCU dynticks counter
atomically in a single operation is a first step towards improving CPU
isolation. This makes the context tracking state updates fully ordered
and therefore allow for later enhancements such as postponing some work
while a task is running isolated in userspace until it ever comes back
to the kernel.
The state field becomes divided in two parts:
1) Lower bits for context tracking state:
CONTEXT_IDLE = 1,
CONTEXT_USER = 2,
CONTEXT_GUEST = 4,
A value of 0 means we are in CONTEXT_KERNEL.
2) Higher bits for RCU eqs dynticks counting:
RCU_DYNTICKS_IDX = 8
The dynticks counting is always incremented by this value.
(state & RCU_DYNTICKS_IDX) means we are NOT in an extended quiescent
state. This makes the chance for a collision more likely between two
RCU dynticks snapshots but wrapping up 24 bits of eqs dynticks
increments still takes some bad luck (also rdp.dynticks_snap could be
converted from int to long?)
Some RCU eqs functions have been renamed to better reflect their broader
scope that now include context tracking state.
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
Cc: Phil Auld <pauld@xxxxxxxxxx>
Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
Cc: Alex Belits <abelits@xxxxxxxxxxx>
---
include/linux/context_tracking.h | 4 +-
include/linux/context_tracking_state.h | 33 ++++++---
kernel/context_tracking.c | 92 +++++++++++++-------------
kernel/rcu/tree.c | 13 ++--
kernel/rcu/tree_stall.h | 2 +-
5 files changed, 81 insertions(+), 63 deletions(-)
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index 63343c34ab4e..aa0e6fbf6946 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -110,7 +110,7 @@ static inline void context_tracking_init(void) { }
#ifdef CONFIG_CONTEXT_TRACKING
extern void ct_idle_enter(void);
extern void ct_idle_exit(void);
-extern unsigned long rcu_dynticks_inc(int incby);
+extern unsigned long ct_state_inc(int incby);
/*
* Is the current CPU in an extended quiescent state?
@@ -119,7 +119,7 @@ extern unsigned long rcu_dynticks_inc(int incby);
*/
static __always_inline bool rcu_dynticks_curr_cpu_in_eqs(void)
{
- return !(arch_atomic_read(this_cpu_ptr(&context_tracking.dynticks)) & 0x1);
+ return !(arch_atomic_read(this_cpu_ptr(&context_tracking.state)) & RCU_DYNTICKS_IDX);
}
#else
diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index bca0d3e0bd3d..b8a309532c18 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -9,13 +9,29 @@
/* Offset to allow distinguishing irq vs. task-based idle entry/exit. */
#define DYNTICK_IRQ_NONIDLE ((LONG_MAX / 2) + 1)
+enum {
+ CONTEXT_IDLE_OFFSET = 0,
+ CONTEXT_USER_OFFSET,
+ CONTEXT_GUEST_OFFSET,
+ CONTEXT_MAX_OFFSET,
+};
+
enum ctx_state {
CONTEXT_DISABLED = -1, /* returned by ct_state() if unknown */
CONTEXT_KERNEL = 0,
- CONTEXT_USER,
- CONTEXT_GUEST,
+ CONTEXT_IDLE = BIT(CONTEXT_IDLE_OFFSET),
+ CONTEXT_USER = BIT(CONTEXT_USER_OFFSET),
+ CONTEXT_GUEST = BIT(CONTEXT_GUEST_OFFSET),
+ CONTEXT_MAX = BIT(CONTEXT_MAX_OFFSET),
};
+/* Even value for idle, else odd. */
+#define RCU_DYNTICKS_SHIFT CONTEXT_MAX_OFFSET
+#define RCU_DYNTICKS_IDX CONTEXT_MAX
+
+#define CT_STATE_MASK (CONTEXT_MAX - 1)
+#define CT_DYNTICKS_MASK (~CT_STATE_MASK)
+
struct context_tracking {
#ifdef CONFIG_CONTEXT_TRACKING_USER
/*
@@ -26,9 +42,8 @@ struct context_tracking {
*/
bool active;
int recursion;
+#endif
atomic_t state;
-#endif
- atomic_t dynticks; /* Even value for idle, else odd. */
long dynticks_nesting; /* Track process nesting level. */
long dynticks_nmi_nesting; /* Track irq/NMI nesting level. */
};
@@ -38,24 +53,26 @@ DECLARE_PER_CPU(struct context_tracking, context_tracking);
static __always_inline int __ct_state(void)
{
- return atomic_read(this_cpu_ptr(&context_tracking.state));
+ return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;
}
static __always_inline int ct_dynticks(void)
{
- return atomic_read(this_cpu_ptr(&context_tracking.dynticks));
+ return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_DYNTICKS_MASK;
}
static __always_inline int ct_dynticks_cpu(int cpu)
{
struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
- return atomic_read(&ct->dynticks);
+
+ return atomic_read(&ct->state) & CT_DYNTICKS_MASK;
}
static __always_inline int ct_dynticks_cpu_acquire(int cpu)
{
struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
- return atomic_read_acquire(&ct->state);
+
+ return atomic_read_acquire(&ct->state) & CT_DYNTICKS_MASK;
}
#endif
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index fe9066fdfaab..87e7b748791c 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -63,9 +63,9 @@ static __always_inline void rcu_dynticks_task_trace_exit(void)
* Increment the current CPU's context_tracking structure's ->dynticks field
* with ordering. Return the new value.
*/
-noinstr unsigned long rcu_dynticks_inc(int incby)
+noinstr unsigned long ct_state_inc(int incby)
{
- return arch_atomic_add_return(incby, this_cpu_ptr(&context_tracking.dynticks));
+ return arch_atomic_add_return(incby, this_cpu_ptr(&context_tracking.state));
}
/*
@@ -74,7 +74,7 @@ noinstr unsigned long rcu_dynticks_inc(int incby)
* RCU is watching prior to the call to this function and is no longer
* watching upon return.
*/
-static noinstr void rcu_dynticks_eqs_enter(void)
+static noinstr void ct_kernel_exit_state(int offset)
{
int seq;
@@ -84,9 +84,9 @@ static noinstr void rcu_dynticks_eqs_enter(void)
* next idle sojourn.
*/
rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
- seq = rcu_dynticks_inc(1);
+ seq = ct_state_inc(offset);
// RCU is no longer watching. Better be in extended quiescent state!
- WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & 0x1));
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & RCU_DYNTICKS_IDX));
}
/*
@@ -94,7 +94,7 @@ static noinstr void rcu_dynticks_eqs_enter(void)
* called from an extended quiescent state, that is, RCU is not watching
* prior to the call to this function and is watching upon return.
*/
-static noinstr void rcu_dynticks_eqs_exit(void)
+static noinstr void ct_kernel_enter_state(int offset)
{
int seq;
@@ -103,10 +103,10 @@ static noinstr void rcu_dynticks_eqs_exit(void)
* and we also must force ordering with the next RCU read-side
* critical section.
*/
- seq = rcu_dynticks_inc(1);
+ seq = ct_state_inc(offset);
// RCU is now watching. Better not be in an extended quiescent state!
rcu_dynticks_task_trace_exit(); // After ->dynticks update!
- WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & 0x1));
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & RCU_DYNTICKS_IDX));
}
/*
@@ -117,7 +117,7 @@ static noinstr void rcu_dynticks_eqs_exit(void)
* the possibility of usermode upcalls having messed up our count
* of interrupt nesting level during the prior busy period.
*/
-static noinstr void rcu_eqs_enter(bool user)
+static noinstr void ct_kernel_exit(bool user, int offset)
{
struct context_tracking *ct = this_cpu_ptr(&context_tracking);
@@ -137,13 +137,13 @@ static noinstr void rcu_eqs_enter(bool user)
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
rcu_preempt_deferred_qs(current);
- // instrumentation for the noinstr rcu_dynticks_eqs_enter()
- instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
+ // instrumentation for the noinstr ct_kernel_exit_state()
+ instrument_atomic_write(&ct->state, sizeof(ct->state));
instrumentation_end();
WRITE_ONCE(ct->dynticks_nesting, 0); /* Avoid irq-access tearing. */
// RCU is watching here ...
- rcu_dynticks_eqs_enter();
+ ct_kernel_exit_state(offset);
// ... but is no longer watching here.
rcu_dynticks_task_enter();
}
@@ -152,7 +152,7 @@ static noinstr void rcu_eqs_enter(bool user)
* ct_nmi_exit - inform RCU of exit from NMI context
*
* If we are returning from the outermost NMI handler that interrupted an
- * RCU-idle period, update ct->dynticks and ct->dynticks_nmi_nesting
+ * RCU-idle period, update ct->state and ct->dynticks_nmi_nesting
* to let the RCU grace-period handling know that the CPU is back to
* being RCU-idle.
*
@@ -189,12 +189,12 @@ noinstr void ct_nmi_exit(void)
trace_rcu_dyntick(TPS("Startirq"), ct->dynticks_nmi_nesting, 0, ct_dynticks());
WRITE_ONCE(ct->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
- // instrumentation for the noinstr rcu_dynticks_eqs_enter()
- instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
+ // instrumentation for the noinstr ct_kernel_exit_state()
+ instrument_atomic_write(&ct->state, sizeof(ct->state));
instrumentation_end();
// RCU is watching here ...
- rcu_dynticks_eqs_enter();
+ ct_kernel_exit_state(RCU_DYNTICKS_IDX);
// ... but is no longer watching here.
if (!in_nmi())
@@ -209,7 +209,7 @@ noinstr void ct_nmi_exit(void)
* allow for the possibility of usermode upcalls messing up our count of
* interrupt nesting level during the busy period that is just now starting.
*/
-static void noinstr rcu_eqs_exit(bool user)
+static void noinstr ct_kernel_enter(bool user, int offset)
{
struct context_tracking *ct = this_cpu_ptr(&context_tracking);
long oldval;
@@ -224,12 +224,12 @@ static void noinstr rcu_eqs_exit(bool user)
}
rcu_dynticks_task_exit();
// RCU is not watching here ...
- rcu_dynticks_eqs_exit();
+ ct_kernel_enter_state(offset);
// ... but is watching here.
instrumentation_begin();
- // instrumentation for the noinstr rcu_dynticks_eqs_exit()
- instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
+ // instrumentation for the noinstr ct_kernel_enter_state()
+ instrument_atomic_write(&ct->state, sizeof(ct->state));
trace_rcu_dyntick(TPS("End"), ct->dynticks_nesting, 1, ct_dynticks());
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
@@ -242,7 +242,7 @@ static void noinstr rcu_eqs_exit(bool user)
/**
* ct_nmi_enter - inform RCU of entry to NMI context
*
- * If the CPU was idle from RCU's viewpoint, update ct->dynticks and
+ * If the CPU was idle from RCU's viewpoint, update ct->state and
* ct->dynticks_nmi_nesting to let the RCU grace-period handling know
* that the CPU is active. This implementation permits nested NMIs, as
* long as the nesting level does not overflow an int. (You will probably
@@ -273,14 +273,14 @@ noinstr void ct_nmi_enter(void)
rcu_dynticks_task_exit();
// RCU is not watching here ...
- rcu_dynticks_eqs_exit();
+ ct_kernel_enter_state(RCU_DYNTICKS_IDX);
// ... but is watching here.
instrumentation_begin();
// instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
- instrument_atomic_read(&ct->dynticks, sizeof(ct->dynticks));
- // instrumentation for the noinstr rcu_dynticks_eqs_exit()
- instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
+ instrument_atomic_read(&ct->state, sizeof(ct->state));
+ // instrumentation for the noinstr ct_kernel_enter_state()
+ instrument_atomic_write(&ct->state, sizeof(ct->state));
incby = 1;
} else if (!in_nmi()) {
@@ -373,22 +373,23 @@ void noinstr __ct_user_enter(enum ctx_state state)
* CPU doesn't need to maintain the tick for RCU maintenance purposes
* when the CPU runs in userspace.
*/
- rcu_eqs_enter(true);
+ ct_kernel_exit(true, RCU_DYNTICKS_IDX + state);
+ } else {
+ /*
+ * Even if context tracking is disabled on this CPU, because it's outside
+ * the full dynticks mask for example, we still have to keep track of the
+ * context transitions and states to prevent inconsistency on those of
+ * other CPUs.
+ * If a task triggers an exception in userspace, sleep on the exception
+ * handler and then migrate to another CPU, that new CPU must know where
+ * the exception returns by the time we call exception_exit().
+ * This information can only be provided by the previous CPU when it called
+ * exception_enter().
+ * OTOH we can spare the calls to vtime and RCU when context_tracking.active
+ * is false because we know that CPU is not tickless.
+ */
+ atomic_add(state, &ct->state);
}
- /*
- * Even if context tracking is disabled on this CPU, because it's outside
- * the full dynticks mask for example, we still have to keep track of the
- * context transitions and states to prevent inconsistency on those of
- * other CPUs.
- * If a task triggers an exception in userspace, sleep on the exception
- * handler and then migrate to another CPU, that new CPU must know where
- * the exception returns by the time we call exception_exit().
- * This information can only be provided by the previous CPU when it called
- * exception_enter().
- * OTOH we can spare the calls to vtime and RCU when context_tracking.active
- * is false because we know that CPU is not tickless.
- */
- atomic_set(&ct->state, state);
}
context_tracking_recursion_exit();
}
@@ -452,15 +453,16 @@ void noinstr __ct_user_exit(enum ctx_state state)
* Exit RCU idle mode while entering the kernel because it can
* run a RCU read side critical section anytime.
*/
- rcu_eqs_exit(true);
+ ct_kernel_enter(true, RCU_DYNTICKS_IDX - state);
if (state == CONTEXT_USER) {
instrumentation_begin();
vtime_user_exit(current);
trace_user_exit(0);
instrumentation_end();
}
+ } else {
+ atomic_sub(state, &ct->state);
}
- atomic_set(&ct->state, CONTEXT_KERNEL);
}
context_tracking_recursion_exit();
}
@@ -530,7 +532,7 @@ void __init context_tracking_init(void)
DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
.dynticks_nesting = 1,
- .dynticks = ATOMIC_INIT(1),
+ .state = ATOMIC_INIT(RCU_DYNTICKS_IDX),
};
EXPORT_SYMBOL_GPL(context_tracking);
@@ -548,7 +550,7 @@ EXPORT_SYMBOL_GPL(context_tracking);
void ct_idle_enter(void)
{
lockdep_assert_irqs_disabled();
- rcu_eqs_enter(false);
+ ct_kernel_exit(false, RCU_DYNTICKS_IDX + CONTEXT_IDLE);
}
EXPORT_SYMBOL_GPL(ct_idle_enter);
@@ -566,7 +568,7 @@ void ct_idle_exit(void)
unsigned long flags;
local_irq_save(flags);
- rcu_eqs_exit(false);
+ ct_kernel_enter(false, RCU_DYNTICKS_IDX - CONTEXT_IDLE);
local_irq_restore(flags);
}
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 90a22dd2189d..98fac3d327c9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -272,9 +272,9 @@ void rcu_softirq_qs(void)
*/
static void rcu_dynticks_eqs_online(void)
{
- if (ct_dynticks() & 0x1)
+ if (ct_dynticks() & RCU_DYNTICKS_IDX)
return;
- rcu_dynticks_inc(1);
+ ct_state_inc(RCU_DYNTICKS_IDX);
}
/*
@@ -293,7 +293,7 @@ static int rcu_dynticks_snap(int cpu)
*/
static bool rcu_dynticks_in_eqs(int snap)
{
- return !(snap & 0x1);
+ return !(snap & RCU_DYNTICKS_IDX);
}
/* Return true if the specified CPU is currently idle from an RCU viewpoint. */
@@ -321,8 +321,7 @@ bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
int snap;
// If not quiescent, force back to earlier extended quiescent state.
- snap = ct_dynticks_cpu(cpu) & ~0x1;
-
+ snap = ct_dynticks_cpu(cpu) & ~RCU_DYNTICKS_IDX;
smp_rmb(); // Order ->dynticks and *vp reads.
if (READ_ONCE(*vp))
return false; // Non-zero, so report failure;
@@ -348,9 +347,9 @@ notrace void rcu_momentary_dyntick_idle(void)
int seq;
raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
- seq = rcu_dynticks_inc(2);
+ seq = ct_state_inc(2 * RCU_DYNTICKS_IDX);
/* It is illegal to call this from idle state. */
- WARN_ON_ONCE(!(seq & 0x1));
+ WARN_ON_ONCE(!(seq & RCU_DYNTICKS_IDX));
rcu_preempt_deferred_qs(current);
}
EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 9bf5cc79d5eb..1ac48c804006 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -459,7 +459,7 @@ static void print_cpu_stall_info(int cpu)
rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
"!."[!delta],
ticks_value, ticks_title,
- rcu_dynticks_snap(cpu) & 0xfff,
+ (rcu_dynticks_snap(cpu) >> RCU_DYNTICKS_SHIFT) & 0xfff ,
ct->dynticks_nesting, ct->dynticks_nmi_nesting,
rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
--
2.25.1